In many situations where fluid, particularly liquid, is screened from a suspension, problems of screen clogging are common. Especially if some of the particles in the material being screened are fine, they have a tendency to clog up the screen slots or openings, resulting in greatly reduced efficiency or stoppage of the screening process.
Problems with screen clogging are especially prevalent in vertical vessels for passing fibrous material suspended in liquid, particularly pulp digesters, such as continuous digesters. Normally the pulp moves downwardly in the vessel under the influence of gravity, and must overcome the wall friction which increases substantially at the screens. Also, slight counter-current flows are provided in parts of the vessel, and there can be a tendency to floatation of the material particles due to the presence of gases. All of these problems, especially the wall friction problem, compound when there is a high content of fines in the pulp material. The result is that the force of gravity may not be sufficient to overcome all of the countervailing forces, and the column of material can stop moving. This is called "hang-up", or "hanging".
Once the material column stops moving, the screens will clog up solidly, and the entire vessel must be shut down until the problem is corrected. Alternatively, there have been numerous installations in the past that have utilized a variety of flow control mechanisms to try to prevent screen clogging, typical conventional flow mechanisms being illustrated in U.S. Pat. Nos. 2,695,232 and 3,711,367. Also, there has been provided a specialized screen having some stationary and some movable parts, the movable parts being pivotal about a generally horizontal axis in response to a horizontal force applied by a cylinder or the like, such as shown in U.S. Pat. No. 3,752,319.
While many prior art proposals can be reasonably effective in preventing hanging, hanging problems still occur in many situations. The present invention deals with the hanging problem, and provides a relatively simple apparatus and method that are utilizable in a wide variety of vessels and environments. The invention is really adaptable to retrofitting to existing installations, and finds particular applicability in both continuous and batch pulp digesting vessels.
The structure according to the present invention is provided in a vertical vessel for vertically passing fibrous material suspended in liquid, the vessel having a vertical wall curved about a vertical axis. The structure includes a vertically elongated screening member, curved about a vertical axis. Preferably a plurality of such members are mounted in the vessel around the interior perimeter thereof, and normally provide for the withdrawal of treatment fluid at various stages of the material processing. Means are provided for mounting the screening members inside and adjacent the vertical wall of the vessel for up and down reciprocal movement, defining a screening volume between the screening member and the vertical wall, and for substantially sealing the volume between the screening member and the vertical wall from the interior of the vessel, except through the screening member and slots of about 2-5 mm clearance between the screen elements and the walls of the vessel, during reciprocation of the screening member. Means are provided for reciprocating the screening member vertically up and down with respect to the vertical wall.
The reciprocating means preferably comprise a single piston and cylinder assembly mounted above each screening member, with the underside of the piston connected to the digester pressure. Means are provided for operatively attaching the piston to the screening member, such as universal joint means, and a fluid conduit is operatively connected to the top of the cylinder and a two-way valve. Means are provided for selectively operatively connecting the valve to a source of fluid under pressure, such as a high pressure liquid pump, or to a vent, such as a flash tank. A hollow component having a plate-like body with a curvature corresponding to the curvature of the screening member may be provided to enclose the reciprocating means and adapt the structure according to the invention to a wide variety of vessels. The body of the hollow component is spaced from the vessel vertical wall at a first end of the body, the first end overlapping the screening member and disposed interiorly thereof, and the body tapers to the vessel wall at a second end thereof opposite the first end. The entire screen assembly may be provided as an insert.
In practicing a method according to the invention, fibrous material, such as paper pulp, is passed vertically downwardly in a vessel, past screens disposed around the vessel periphery. The screens are reciprocated in the same direction of movement as the pulp, proper cleaning of the screens resulting. The screens are not moved positively by an actuator in both directions. Rather the screens are only moved downwardly by an actuator, but are allowed to move upwardly themselves under digester pressure.
It is the primary object of the present invention to provide a simple and effective structure for keeping screens in digesters, or the like, from clogging. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.